Terminal insulator



Dec. 8, 1959 R. s. Toms 2,916,719

TERMINAL INSULATOR Filed Jan. 9, 1956 nv VEN TOR R. S. TOMS ATTORNEY United States Patent TERMINAL INSULATOR Application January 9, 1956, Serial No. 557,902

9 Claims. (Cl. 339-60) This invention relates to insulators and, more particularly, to corona prevention insulators to be used on electron tube terminals or other high voltage terminals.

Corona is a luminous discharge which is due to ionization of the air surrounding a conductor around which exists a voltage gradient exceeding a certain critical value. Corona initially take place at a sharp edge, corner or other abrupt change in the surface of the conductor. It has been found that corona increases with increasing altitude, which increase is apparently due to the increase in the natural ionization which exists at higher altitudes. The effect of this condition is to increase the equivalent voltage of the equipment and may increasethe equivalent voltage beyond the critical value at which corona occurs. When corona begins to occur, ozone is formed in the immediate area. This ozone represents a high resistance path for electrical currents, and if sufiicient ozone is formed to establish a path between the high voltage terminal and another point of lower potential on the equipment, such as the grounded chassis, then current flows through the ozone and more ozone is formed until an arc is formed effectively shortcircuiting the high voltage terminal. Thus, under extreme conditions, corona results in complete circuit failure. Consequently, corona becomes an appreciable problem in airborne electronic equipment, in nonpressurized locations, utilizing high voltages. For example, it has been found that electronic equipment which is operative at low altitudes may be inoperative at high altitudes in nonpressurized regions as the equivalent voltage is effectively greatly increased.

Accordingly, it is an object of this invention to eliminate corona discharge.

It is a further object of this invention to provide improved high voltage anode terminal insulation on electron tubes.

It is a further object of this invention to permit higher I voltages to be used on electron tubes having cap type terminals.

Briefly, in accordance with aspects of this invention, an annular resilient insulating member is detachably positioned between a terminal of an electron discharge device and a terminal cap, the insulating member having such configuration as to provide a substantially air-tight seal between the base of the terminal and the terminal cap. This bushing has one shape before it is engaged with the terminal of the electron tube and another shape after being engaged with the tube terminal with the terminal cap attached to the terminal. The bushing has an enlarged portion and a reduced portion of smaller diameter integral therewith. Before being engaged with the terminal, the enlarged portion lies in a plane parallel to the plane of the reduced portion. When the bushing is engaged with the terminal, the innermost portion of the bushing grips the surface of the terminal while the pres sure exerted by the terminal cap as well as the configuration of the inner surface and gripping action of the bushing cause the bushing to fold over. The enlarged portion folds over the reduced portion and the lower position of both portions is determined by the surface of the tube envelope. In its final position, the enlarged portion lies on the same plane as the reduced portion and forms a substantially airtight seal between the lower surface of the terminal cap and the tube envelope. Advantageously, the reduced portion forms a tight seal at the base of the tube terminal, which is the conductor surface irregularity at which corona is most likely to occur.

In accordance with another aspect of this invention the upper and lower portions are specifically shaped to facilitate the folding over of the enlarged portion onto the reduced portion and thus to ensure the formation of the substantially airtight seal between the terminal cap and the base of the terminal. Specifically, in one illustrative embodiment of this invention, the two portions of the bushing are both ring-shaped with their inner surfaces defining frustums of cones having their planes of least diameter adjacent each other. Further, the minimum diameter of the inner surface of the upper portion of the bushing is greater than the minimum diameter of the inner surface of the lower portion of the bushing.

By this specific structural arrangement, the positioning of the bushing on the terminal, in accordance with this invention, is facilitated. Specifically, the inner surface of the lower portion of the bushing closely fits onto the terminal and in the final position of the bushing the lower portion of the bushing against the terminal is actually above the upper portion of the bushing, which upper portion has been depressed and folded over by the action of the terminal cap. At the same time, because of the shape of the inner surface of the upper portion of the bushing, the upper portion readily folds over the lower portion and forms a substantially airtight seal between the bushing and the tube envelope.

It is therefore apparent that any leakage path from the base of the terminal along the tube envelope would have to include both the lower bushing portion, which is closely fitted onto the terminal and, because of the shape of its inner surface, also partially onto the tube envelope, and the upper bushing portion which has made a substantially airtight seal to the tube envelope. In this manner the formation of corona around the exposed base of the anode terminal is effectively prevented as the ambient or surrounding air is excluded from the terminal base and is in fact replaced by a medium, namely the insulating bushing, having a higher dielectric constant. By preventing corona from forming, ionization of the air around the terminal base and eventual arcing breakdown are also eflfectively prevented, as discussed further above.

Accordingly, it is a feture of this invention to engage an electron tube terminal with a resilient annular bushing Having a reduced portion adapted to engage the base of the terminal and an enlarged portion adapted to engage the terminal cap and thus form an airtight seal between the cap and terminal base.

It is another feature of this invention that the annular resilient insulating bushing have intersecting frustums of conical surfaces on the inner surface of the bushing, the first frustum being adapted to form a snug fitting seal with the base of the tube terminal while the second frustum is adapted to ensure a fold-over action when the terminal cap is pressed against the upper surface of the bushing.

It is another feature of this invention to engage a tube terminal with a bushing having an inner diameter less than the outer diameter of the tube terminal and depress the bushing with a terminal cap to cause the bushing effectively to fold over and form an airtight seal between the terminal cap and the terminal.

A complete understanding of this invention and of these and various other features thereof may be gained from consideration of the following detailed description and the accompanying drawing in which:

Fig. 1A is a bottom view of a bushing in accordance with one specific illustrative embodiment of this invention;

Fig. 1B is a cutaway section of the bushing of Fig. 1A along the lines 1B1B thereof;

Fig. 2A depicts a tube terminal having a bushing, as depicted in Fig. 1A, initially engaged thereon;

Fig. 2B depicts a first bushing in accordance with this invention engaged with the base of the terminal with the cap removed and a second bushing in accordance with this invention engaged with the base of the terminal with the cap attached; and

Fig. 3 is an enlarged sectional view of the righthand terminal and cap connection of Fig. 23.

Referring noW to Fig. 1A, there is depicted a bushing having an enlarged circular portion 1t) and a reduced circular portion 11. The inner surfaces of the bushing are defined by a frustum of a conical surface 12 for the enlarged portion and a frustu'm of a conical surface 13 for the reduced portion as best seen in Fig. 1B. The bushing may be of any of various resilient insulating materials known in the art, such as various types of rubber including silicone rubber. Specifically, I have found that a silicone rubber material with a dielectric constant great er than that of air may advantageously be employed.

Fig. 2A depicts a bushing in accordance with this invention engaging a terminal 16 of a vacuum tube envelope 17 in which the reduced portion 11 of the bushing is placed downwardly in a direction to engage the base of terminal 16.

The left-hand portion of Fig. 2B depicts such a bushing on the lower portion of the terminal in which the lower edge of reduced portion 11 surrounds the base of terminal 16 and touches the surface of tube envelope 17 while an insulating tube terminal cap 19 is positioned to engage terminal 16 and the upper surface of enlarged portion 10.

The right-hand portion of Fig. 2B depicts terminal cap 19 engaged with terminal 16 and pressed to its lowest position thus compressing the bushing and causing the en larged portion to fold over reduced portion 11 to such a degree that enlarged portion 10 engages the surface of envelope 17 and is retained in substantially the same plane as reduced portion 11.

Fig. 3 depicts a cutaway of the right-hand connection of Fig. 2B and particularly illustrates the configuration of this specific embodiment of the invention when in position on the tube terminal. As can be seen in the drawing, the lower surface of enlarged portion 16 engages the surface of the tube envelope 17 while the upper surface of portion 10 engages the edge of insulating tube terminal cap 19. The pressure exerted by cap 1& on enlarge-d portion 10 has caused this portion to fold over reduced portion 11. This fold-over is also aided by the configuration of the inner surface 12 in accordance with an aspect of my invention. As this surface advantageously defines a frustum of a cone, upper enlarged portion 16 presents only a small opposing force to cap 19 while the innermost part of surface 13 grips the surface of terminal 16. Surface 13 engages the base of terminal 16 in snug fitting relationship. A cap spring contactor 2ft is advantageously positioned between the terminal 16 and the terminal cap 19 and performs a dual function in that it makes electrical contact with terminal 16 and by frictionally engaging this terminal effectively locks the terminal cap in compressive contact with the upper surface of the bushing.

Thus, the bushing having the novel configuration in accordance with my invention, by engaging the base of terminal 16 in snug fitting relationship and by the fold-over action of enlarged portion 10 over reduced portion 11, provides a substantially airtight seal between the insulating tube cap 19 and the electron tube envelope 17, thereby preventing corona discharge from the terminal.

in one specific illustrative embodiment of this invention, as depicted in the drawing, the enlarged portion 10 has an outer diameter of three-fourths of an inch while the conical inner surface 12 of this portion is developed at an angle of 30 degrees with respect to the axis of the bushing; the least inner diameter of the inner surface 12 is three-eighths of an inch. The reduced portion 11 has an outer diameter of seven-sixteenths of an inch and a least inner diameter of five-sixteenths of an inch; the conical inner surface 13 of the reduced portion subtends an angle of degrees with respect to the axis of the bushing. The thickness of the enlarged portion is one eighth of an inch while the thickness of the bushing including the enlarged portion and the reduced portion is three-sixteenths of an inch. This specific bushing is designed to engage a tube terminal having approximately three-eighths of an inch outer diameter. Thus, the least diameter of the inner surface of the bushing must be stretched one-sixteenth of an inch in order to surround the tube terminal. The terminal cap has an inner diameter somewhat less than the three-fourths of an inch outer diameter of the enlarged portion but the inner diameter of this terminal cap is greater than the seven-sixteenths of an inch outer diameter of the reduced portion.

it is to be understood that the above-described arrangements are illustrative of the application of the principles of the invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. An electron tube comprising a tube envelope and an anode terminal on the surface of said envelope, an anode terminal cap on said terminal, an anode terminal corona prevention insulating bushing of resilient material intermediate said tube envelope and said anode terminal cap, said bushing comprising an annular ring having upper and lower portions, said upper portion being of larger outside diameter than the outer diameter of the terminal cap and said lower portion being of an outside diameter less than the inside diameter of the terminal cap, said lower portion surrounding and being in snug fitting relationship with and having an inside diameter substantially equal to the diameter of said terminal, said terminal cap being positioned on said upper portion of said bushing and exerting pressure thereon causing said upper portion to fold over said lower portion, and means causing said terminal cap to maintain its position on said terminal and to exert pressure downward on said upper portion of said bushing thereby forming a substantially air-tight seal between said cap and the base of said terminal.

2. An electron tube according inner surface of said upper and define frustums of cones having adjacent.

3. An electron tube according to claim 1 wherein the inner surfaces of said upper and lower. annular portions define frustums of cones having their smaller diameters in the same plane.

4-. An electron tube according to claim 3 wherein the smaller diameter of said frustum of said upper portion is less than said inner diameter of said terminal cap, and the larger diameter of said frustum of said upper portion is larger than said outer diameter of said terminal cap, said upper portion frustum forming a webbed portion at the intersection with said lower portion thereby facilitating the folding of said upper portion about said webbed portion when pressure is exerted by said cap.

5. in combination, an electron discharge device having an envelope and a terminal extending through said envelope, a resilient corona prevention annular insulating bushing on said terminal, said bushing having an to claim 1 wherein the lower annular portions their smaller diameters upper annular portion and a lower annular portion, the outer diameter of said upper portion larger than the outer diameter of said lower portion and said lower portion surrounding and being in snug fitting relationship with said terminal, and means causing pressure to be exerted downward on said upper portion toward the base of said terminal thereby causing said upper portion to fold over said lower portion with the lower face of said upper portion against said envelope forming a substantially airtight seal between said bushing and said envelope.

6. The combination according to claim 5 wherein the inner surfaces of said upper and lower annular portions define frustums of cones having their smaller diameters adjacent.

7. The combination according to claim 6 wherein said smaller diameters of said frustums are in the same plane.

8. The combination according to claim 7 wherein the means causing downward pressure to be applied comprises a member bearing against an upper face of said upper portion of said bushing at a plurality of points on a circumference of greater diameter than the outer diameter of said lower portion thereby facilitating the folding of said upper portion over said lower portion.

9. A corona prevention bushing for attachment to terminals of electron discharge devices comprising a resilient ring of insulating material having a first portion and a second portion disposed along the axis of said ring, said first and second portions each having an inner surface and an outer surface, said inner surfaces of said portions defining frustums of cones having their smallest diameters in a common plane, the smallest diameter of said frustum of said first portion being less than the smallest diameter of said frustum of said second portion and such as to form a snug fit around said terminals, said outer surface of each of said portions defining substantially a cylinder terminating in said common plane, said outer surface of said second portion having a diameter larger than said outer surface of said first portion to provide a thin annular portion lying in said common plane operative as a fulcrum for folding said second portion over said first portion.

References Cited in the file of this patent UNITED STATES PATENTS 1,703,725 Cromwell Feb. 26, 1929 2,228,138 Larkin Jan. 7, 1941 2,269,296 Weaver Jan. 6, 1942 2,322,491 Williams June 22, 1943 2,367,512 MacFadden Jan. 16, 1945 2,673,751 Finch Mar. 30, 1954 2,690,470 Moorhead Sept. 28, 1954 2,707,723 Moorhead May 3, 1955 

